During combustion, an internal combustion engine oxidizes gasoline and combines hydrogen (H2) and carbon (C) with air. Combustion creates chemical compounds such as carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds. During an initial startup period after a long soak, the engine is still “cold” after starting and combustion of the gasoline is incomplete. A catalytic converter treats exhaust gases from the engine. During the startup period, the catalytic converter is also “cold” and does not operate optimally.
In one conventional approach, an engine controller commands a lean air/fuel (A/F) ratio and supplies a reduced mass of liquid fuel to the engine to provide compensation. More air is available relative to the mass of liquid fuel to sufficiently oxidize the CO and HC. However, the lean condition reduces engine stability and adversely impacts vehicle drivability.
In another conventional approach, the engine controller commands a fuel-rich mixture for stable combustion and good vehicle drivability. A secondary air injection system provides an overall lean exhaust A/F ratio by injecting air into the exhaust stream during the initial start-up period. The additional injected air heats the catalytic converter due to the exothermic reaction of oxidizing the excess CO and HC. The warmed catalytic converter oxidizes CO and HC and reduces NOx to lower emissions levels.
This approach, however, includes distinct disadvantages. One disadvantage is that the secondary air injection system increases cost and complexity of the engine control system and is only used during a short initial cold start period. Another disadvantage is that the additional liquid fuel produces a fuel film that coats the engine components and contributes to uncontrolled HC emissions, oil contamination, spark ignition problems and increased fuel consumption.